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Substituting text by regular expression

Global substitution in a string is done by patsubst:

patsubst(string, regexp, opt replacement)

which searches string for matches of regexp, and substitutes replacement for each match. The syntax for regular expressions is the same as in GNU Emacs.

The parts of string that are not covered by any match of regexp are copied to the expansion. Whenever a match is found, the search proceeds from the end of the match, so a character from string will never be substituted twice. If regexp matches a string of zero length, the start position for the search is incremented, to avoid infinite loops.

When a replacement is to be made, replacement is inserted into the expansion, with `\n' substituted by the text matched by the nth parenthesized sub-expression of regexp, `\&' being the text the entire regular expression matched.

The replacement argument can be omitted, in which case the text matched by regexp is deleted.

patsubst(`GNUs not Unix', `^', `OBS: ')
=>OBS: GNUs not Unix
patsubst(`GNUs not Unix', `\<', `OBS: ')
=>OBS: GNUs OBS: not OBS: Unix
patsubst(`GNUs not Unix', `\w*', `(\&)')
=>(GNUs)() (not)() (Unix)
patsubst(`GNUs not Unix', `\w+', `(\&)')
=>(GNUs) (not) (Unix)
patsubst(`GNUs not Unix', `[A-Z][a-z]+')
=>GN not 

Here is a slightly more realistic example, which capitalizes individual word or whole sentences, by substituting calls of the macros upcase and downcase into the strings.

define(`upcase', `translit(`$*', `a-z', `A-Z')')dnl
define(`downcase', `translit(`$*', `A-Z', `a-z')')dnl
     `regexp(`$1', `^\(\w\)\(\w*\)', `upcase(`\1')`'downcase(`\2')')')dnl
     `patsubst(`$1', `\w+', `capitalize1(`\&')')')dnl
capitalize(`GNUs not Unix')
=>Gnus Not Unix

The builtin macro patsubst is recognized only when given arguments.

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